Nonsteady-State Kinetics of Droplet Growth in Cloud Physics

Norbert Nix Denver Research Institute, University of Denver, Colorado 80210

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Norihiko Fukuta Denver Research Institute, University of Denver, Colorado 80210

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Abstract

The nonsteady-state kinetics of droplet growth or evaporation is studied theoretically for two types of changing boundary conditions: changes in the distant environment, and changes at the droplet surface. The treatment assumes Maxwellian boundary conditions at the droplet surface. The nonsteady-state kinetics during rapid environmental changes is reviewed, and compared with the simulation of the process obtained by applying the quasi-steady-state theory in a stepwise manner. A method for estimating the errors arising from use of the quasi-steady-state theory to determine temperature and vapor fields and growth or evaporation rates is explained. The nonsteady-state kinetics for changing surface conditions is treated, taking a freezing supercooled droplet as an example. It is shown that a freezing supercooled droplet generates a wave of supersaturation around it. The head of the wave moves outward to a position about one droplet diameter away from the surface, and remains there until the droplet is completely frozen. Satellite droplet formation around a freezing droplet is explained by nucleation on unactivated condensation nuclei under the influence of this supersaturation. It is suggested that the supersaturation near a droplet which freezes may activate nearby ice nuclei which would be inactive under normal environmental conditions in a cloud.

Abstract

The nonsteady-state kinetics of droplet growth or evaporation is studied theoretically for two types of changing boundary conditions: changes in the distant environment, and changes at the droplet surface. The treatment assumes Maxwellian boundary conditions at the droplet surface. The nonsteady-state kinetics during rapid environmental changes is reviewed, and compared with the simulation of the process obtained by applying the quasi-steady-state theory in a stepwise manner. A method for estimating the errors arising from use of the quasi-steady-state theory to determine temperature and vapor fields and growth or evaporation rates is explained. The nonsteady-state kinetics for changing surface conditions is treated, taking a freezing supercooled droplet as an example. It is shown that a freezing supercooled droplet generates a wave of supersaturation around it. The head of the wave moves outward to a position about one droplet diameter away from the surface, and remains there until the droplet is completely frozen. Satellite droplet formation around a freezing droplet is explained by nucleation on unactivated condensation nuclei under the influence of this supersaturation. It is suggested that the supersaturation near a droplet which freezes may activate nearby ice nuclei which would be inactive under normal environmental conditions in a cloud.

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